REV. A

OP285

–11–

A 3-Pole, 40 kHz Low-Pass Filter

The closely matched and uniform ac characteristics of the OP285

make it ideal for use in GIC (Generalized Impedance Converter)

and FDNR (Frequency Dependent Negative Resistor) filter appli-

cations. The circuit in Figure 16 illustrates a linear-phase,

3-pole, 40 kHz low-pass filter using an OP285 as an inductance

simulator (gyrator). The circuit uses one OP285 (A2 and A3)

for the FDNR and one OP285 (Al and A4) as an input buffer

and bias current source for A3. Amplifier A4 is configured in a

gain of 2 to set the pass band magnitude response to 0 dB. The

benefits of this filter topology over classical approaches are

that the op amp used in the FDNR is not in the signal path and

that the filter’s performance is relatively insensitive to compo-

nent variations. Also, the configuration is such that large signal

levels can be handled without overloading any of the filter’s

internal nodes. As shown in Figure 17, the OP285’s symmetric

slew rate and low distortion produce a clean, well-behaved

transient response.

10

90

100

0%

SCALE: VERTICAL – 2V/ DIV

HORIZONTAL – 10 S/ DIV

10V p-p

10kHz

Figure 17. Low-Pass Filter Transient Response

3

2

1

A1

A1, A4 = 1/2 OP285

A2, A3 = 1/2 OP285

1

A2

2

3

R1

95.3k

C1

2200pF

R2

787

C2

2200pF

R3

1.82k

C3

2200pF

R4

1.87k

R5

1.82k

A3

5

7

6

A4

5

7

6

R6

4.12k

R7

100k

R9

1k

R8

1k

C4

2200pF

Figure 16. A 3-Pole, 40 kHz Low-Pass Filter

Driving Capacitive Loads

The OP285 was designed to drive both resistive loads to 600

Ω

and capacitive loads of over 1000 pF and maintain stability. While

there is a degradation in bandwidth when driving capacitive loads,

the designer need not worry about device stability. The graph in

Figure 18 shows the 0 dB bandwidth of the OP285 with capacitive

loads from 10 pF to 1000 pF.

0

0

200

400

600

800

1000

1

2

3

4

5

6

7

8

9

10